It is known that when a coating film showing a refractive index lower than the refractive index of a substrate is formed on the surface of the substrate, the reflectance of light reflected from the surface of the coating film decreases. Such a coating film showing a decreased light reflectance is utilized as an antireflection film and practically applied to various substrate surfaces.
A process for forming an antireflection film having a low refractive index, on a substrate, is disclosed which comprises applying on a glass substrate such as a cathode ray tube an alcohol dispersion of fine particles of MgF2 formed by reacting a magnesium salt or an alkoxy magnesium compound as a Mg source with a fluoride salt as a F source, or a liquid having tetraalkoxysilane or the like added thereto for improving the film strength, as a coating fluid, followed by heat-treatment at a temperature of from 100° C. to 500° C. (Patent Document 1).
A low reflection glass having formed on a glass substrate a thin film showing a refractive index of from 1.21 to 1.40 and having a thickness of from 60 to 160 nm with irregularities or micro-pits having a diameter of from 50 to 200 nm, is disclosed, wherein the film is formed by mixing a solvent such as an alcohol with at least two hydrolytic polycondensates different in the average molecular weight, such as tetraalkoxysilane, methyltrialkoxysilane, to obtain a coating fluid, forming a coating film from such a coating fluid by controlling the relative humidity and the mixing ratio at the time of the above mixing, and heating the coating film (Patent Document 2).
A low reflectance glass comprising a glass, a lower layer film having a high refractive index formed on its surface and an upper layer film having a low refractive index formed on the surface thereof, is disclosed (Patent Document 3). As a detailed description of the process for forming the upper layer film, this publication discloses a process which comprises hydrolyzing a fluorine-containing silicone compound having a polyfluorocarbon chain such as CF3(CF2)2C2H4Si(OCH3)3 and a silane coupling agent such as Si(OCH3)4 in an amount of from 5 to 90 wt % based thereon, in an alcohol solvent in the presence of a catalyst such as acetic acid, followed by filtration to obtain a liquid of a co-polycondensate, then applying this liquid on the lower layer film and heating it at a temperature of from 120 to 250° C.
A coating film having a refractive index of from 1.28 to 1.38 and a contact angle with water of from 90° to 115°, is disclosed which is formed as adhered on a substrate surface, by preparing a reaction mixture comprising a silicon compound of the formula Si(OR)4, a silicon compound of the formula CF3(CF2)nCH2CH2Si(OR1)3, an alcohol of the formula R2CH2OH and oxalic acid in a specific ratio, heating this reaction mixture at a temperature of from 40 to 180° C. in the absence of water to form a solution of a polysiloxane, applying a coating fluid containing the solution on a substrate surface to form a coating, and heat-curing the coating at a temperature of from 80 to 450° C. (Patent Document 4).    Patent Document 1: JP-A-05-105424    Patent Document 2: JP-A-06-157076    Patent Document 3: JP-A-61-010043    Patent Document 4: JP-A-09-208898